1041 All Air Systems

The most common types of systems for heating and cooling buildings are those which moderate the air temperature of the occupied space by providing a supply of heated or cooled air from a central source via a network of air ducts. These systems, referred to as all-air systems, increase or decrease the space temperature by altering either the volume or temperature of the air supplied.

Recalling that the most important determinant of thermal comfort in a warm environment is air velocity, most buildings which require cooling employ all-air systems. Consequently, all-air systems are the system of choice when cooling is required. All-air systems also provide the best control of outside fresh air, air quality, and humidity control. An added benefit of forced air systems is that they can often use outside air for cooling interior spaces while providing heating for perimeter spaces. (See §10.5.5; Economizers) The advantages of all-air systems are offset somewhat by the energy consumed in distribution.

All-air systems tend to be selected when comfort cooling is important and for thermally heavy buildings which have significant internal cooling loads which coincide with heating loads imposed by heat loss through the building envelope.

The components of an all-air HVAC system include an air-handling unit (AHU) which includes a fan, coils which heat and/or cool the air passing through it, filters to clean the air, and often elements to humidify the air. Dehumidification, when required, is accomplished by cooling the air below the dew-point temperature. The conditioned air from the AHU is supplied to the occupied spaces by a network of supply-air ducts and air is returned from conditioned spaces by a parallel network of return-air ducts. (Sometimes the open plenum above a suspended ceiling is used as part of the return-air path.) The AHU and its duct system also includes a duct which supplies fresh outside air to the AHU and one which can exhaust some or all of the return air to the outside. Figure 10.3 depicts the general arrangement of components in an all-air HVAC system.